1. Field of the Invention
This invention relates to a method for producing sialon powders, particularly .beta.-sialon powders.
2. Description of the Related Art
.beta.-sialon is a substitution type solid solution in which some silicon sites are occupied by aluminum and some nitrogen sites are occupied by oxygen in a .beta.-silicon nitride structure. The term sialon refers comprehensively to compounds and/or solid solutions composed of the Si,Al,O and N elements, but usually the term "sialon" merely refers to .beta.-sialon, which is represented by the formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z (wherein z is a number greater than 0 and not greater than 4.2). In the present specification, sialon means .beta.-sialon unless otherwise specifically noted.
Sialon has a high resistance to oxidation at a high temperature and also an excellent corrosion resistance to molten metals or slag, and therefore, sialon powders are useful as the starting material for various refractory materials. Also, sialon ceramics obtained by sintering sialon powders have hiqh strength and hardness at a high temperature and are excellent for use in engine parts, gas turbine parts or cutting tools.
As a method for producing sialon powders, for example, the following methods are known.
(1) The method in which carbon is added to a clay mineral such as kaolinite, etc., which is a naturally occurring silica-alumina type mineral, and the mixture is subjected to carbothermal reduction and nitridation by heating in a nitrogen gas stream [Journal of Ceramic Society of Japan, 91, 442 (1983)]. PA0 (2) The method in which aluminum metal is added to a silica source such as shirasu, etc., and the mixture is heated in a nitrogen gas stream. PA0 (3) The method in which a co-precipitate of aluminum hydroxide and silica gel is heated in an ammonia atmosphere [American Ceramic Society Bulletin, 58, 191 (1979)]. PA0 (4) The method in which carbon powder is uniformly dispersed in a silica-alumina mixture obtained by hydrolysis of a powder mixture of silicon alkoxide and aluminum alkoxide, and the dispersion is heated in a nitrogen gas atmosphere (Japanese Unexamined Patent Publication (Kokai) No. 60-145902).
The above methods (1) and (2) have a drawback in that a large amount of impurities remain in the sialon powders because natural occurring starting materials are used. The above method (3) has a drawback in that unaltered reactants remain in the sialon powders produced because the reaction is not likely to be completed. According to method (4), fine particle sialon powders having very high purity can be obtained, but there are still drawbacks such that unaltered carbon remains [see Journal of Ceramic Society of Japan, 93, 69 (1985)]or that it is difficult to obtain sialon powders with any desired composition [see Journal of Ceramic Society of Japan, 93, 34 (1985)], and therefore, it is not satisfactory as an industrial production method from the aspect of the starting material and process.